Join Date: Nov 2000
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Re: Ported vacumn
Manifold vacuum is taken from, well, the manifold. Which means that when the motor is running, the less throttle you give it, the more vacuum is created in the manifold by the engine. As you open the throttle, there is less restriction to air flow into the manifold, and vacuum drops off. In some scenario's, like driving in a given gear, less vacuum means more load on the engine. Power brakes and tvac (the heat gate controlling warm air into your air cleaner) also run off manifold vacuum. They lose their power source, vacuum, at wot. Then again, who needs brakes at wot?
Ported vacuum is taken from within the carb's venturi, above the throttle plates. As you open the throttle, it rises quickly, as manifold vacuum drops, then eventually as wide open throttle (wot), both drop off. Most stock distributor's vacuum advance run off ported vacuum. More throttle, more advance, to a point. Some are hooked up to a combo of the two depending on how warm the engine is. You tune your idle circuit of your carb by watching manifold vacuum, as the higher the vacuum, the stronger the motor is running in proportion to the small amount the throttle is open for idle operation. Hope this helps.
84 CJ7, 258, HEI, M/C 2100 carb, 5 inch lift, RS9000's, 33x12.5 BFG M/T's, 4.56's and Detroit softlocker, full cage & belts, Xenon flares, Dana 44 rear, GM dual diaphragm brake booster